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bpf: Zero-fill other CPUs when BPF_F_CPU creates a per-cpu hash element

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pcpu_init_value() initializes the per-cpu area of a newly created
[lru_]percpu_hash element.  The area is recycled, so when the value
comes from a BPF program (onallcpus == false) it writes the running
CPU's slot and zeroes the rest.

bpf_percpu_hash_update() passes onallcpus == true, which delegates to
pcpu_copy_value().  pcpu_copy_value() writes only the CPU named in
map_flags when BPF_F_CPU is set, so on the create path the other slots
keep the recycled element's values:

  update(k1, 0xdeadc0de, BPF_F_ALL_CPUS)  every CPU holds 0xdeadc0de
  delete(k1)                              element back on the freelist
  update(k2, 0xc0ffee, BPF_F_CPU | 0)     creates, writes CPU 0 only
  lookup(k2)                              CPU 0 0xc0ffee, rest 0xdeadc0de

Zero-fill the other CPUs on that arm too.

Fixes: c6936161fd ("bpf: Add BPF_F_CPU and BPF_F_ALL_CPUS flags support for percpu_hash and lru_percpu_hash maps")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20260924102321.2120434-2-donggeunyoo.kernel@gmail.com
This commit is contained in:
Donggeun Yoo 2026-09-24 19:23:20 +09:00 committed by Alexei Starovoitov
parent 8110ba0977
commit c3a66e5f5b
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1 changed files with 7 additions and 4 deletions
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11
kernel/bpf/hashtab.c
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@ -1054,14 +1054,17 @@ static void pcpu_init_value(struct bpf_htab *htab, void __percpu *pptr,
/* When not setting the initial value on all cpus, zero-fill element
* values for other cpus. Otherwise, bpf program has no way to ensure
* known initial values for cpus other than current one
* (onallcpus=false always when coming from bpf prog).
* (onallcpus=false always when coming from bpf prog,
* map_flags & BPF_F_CPU when coming from syscall but setting
* only one cpu).
*/
if (!onallcpus) {
int current_cpu = raw_smp_processor_id();
if (!onallcpus || (map_flags & BPF_F_CPU)) {
int init_cpu = (map_flags & BPF_F_CPU) ? map_flags >> 32 :
raw_smp_processor_id();
int cpu;
for_each_possible_cpu(cpu) {
if (cpu == current_cpu)
if (cpu == init_cpu)
copy_map_value(&htab->map, per_cpu_ptr(pptr, cpu), value);
else /* Since elem is preallocated, we cannot touch special fields */
zero_map_value(&htab->map, per_cpu_ptr(pptr, cpu));